Observations of Obscured Black Holes Yoshihiro Ueda (Department of Astronomy, Kyoto University)

Search for obscured black holes Nearly every present-day galaxy contains a BH in its centre with a mass proportional to the spheroid mass, indicating a tight link between the BH and star formation: SMBH is a key ingredient of the universe Most AGNs are “obscured” (cannot always be distinguished or recognized in other wavelengths). Hard X-ray observations are the most straightforward approach to detect this population without selection biases In fact, massive star forming galaxies contain rapidly growing BHs heavily obscured by dust (submilimeter galaxies at z~2, Alexander et al. 2005; ULIRGs at z~0, Imanishi et al. 2006). This is consistent with the “co-evoluton” scenario.

Star forming history vs Co-evolution of galaxy and super massive black holes in galactic centres BH mass vs Stellar mass @z=0 Star forming history vs accretion history e.g., Marconi & Hunt 03 Marconi+ 04

The X-Ray Background (XRB) The XRB is the integrated eission from all the AGNs in the universe, telling us the formation history of supermassive black holes. The energy density peaks at ~ 30 keV The shape of the XRB indicates that most of the AGNs are obscured (such as Seyfert 2; Awaki et al. 1993) The 2-10 keV band is much better than 0.5-2 keV, but 10-100 keV is the best energy band to detect obscured AGNs including Compton thick AGNs Comastri+ 95

X-ray Spectra of Heavily Obscured AGNs Compton thick AGNs: NH>1024 cm-2 show complex spectra as a function of column density Reflection/scattered component can be detected below 10 keV but only limited information can be drawn (e.g.,no intrinsic luminosity) Wilman & Fabian (1999) Done+ (2003) NGC 4945 Log NH=24.25 Log NH=24.75 Log NH=25.25

What are known from X-ray surveys below 10 keV Subaru-XMM Deep Survey fields Log N log S relations (2-10 keV) 1 deg Kushino+ 02

Sample: 1371 AGNs survey N flux limit reference of optical ID HEAO-1 49 1.7x10-11 Piccinotti+82, Grossan+82 ASCA 142 3x10-14 Akiyama+00, Akiyama+03, Ishisaki+01 Chandra 160 1.1x10-15 Barger+03, Szokoly+04, Zheng+ 04 ROSAT/XMM/Chandra 1020 1.1x10-16 Hasinger+ 05 and therin

(1)+(2) → Population Synthesis Model The current status of X-ray surveys The XRB below ~ 6 keV has been almost completely resolved and identified and hence is well understood. Howerver, even with the deepest Chandra/XMM surveys a significant fraction of the XRB remains unresolved above 6 keV (Worsley et al. 2004). Above 10 keV only 1 percent of the XRB is resolved at present. Extrapolation has to be made beyond the observational results. Population synthesis model reproducing the XRB spectrum Given the luminosity function and absorption function determined below 10 keV, we predict contribution of Compton-thin AGNs to the background above 10 keV with an assumption of a broad band spectrum over 0.5-1000 keV The missing background is then be attributed to Compton thick AGNs

Population synthesis model The observed XRB spectrum Integrated AGN emission from our HXLF and the absorption function (lower black) well reproduces the broad band XRB spectrum (blue) below 300 keV High energy cutoff must be arround 500 keV in average (red left: 400 keV, red right: 600 keV) Presense of Compton-thick AGNs estimated by Risaliti et al (1999) is consistent with the XRB spectrum (upper black) Reflection components are important (green: no reflection) Ueda+ 2003

Compton thick AGNs or Compton reflection? The fraction of Compton thick AGNs, introduced to reproduce the intensity XRB spectrum at 30 keV, is coupled with the amount of reflection component (assumed to be Ω=2πfor both type-1 and type-2 AGNs) Precise study of broad band spectra of neaby AGNs (especially type-2 AGNs) is crucial. Suzaku observations are important. Integrated spectrum of type-1 AGNs Compton-thick AGNs 0.5 1 10 100 (keV) Observed XRB spectrum Ueda+ 2003

A big remaining issue: The number density of Compton thick AGNs Significant contribution to the SMBH mass growth? “AGN relic” black hole mass function can be calculated from the luminosity function of the “whole AGNs” including Compton thick ones For instance, Marconi et al. (2004) have to assume 0.6 times additional Compton thick AGNs as many as Compton thin ones to reproduce the local BH mass function In the local universe the number density of Compton thick AGNs may be comparable to or even larger than Compton thin AGNs (Maiolino et al. 2003) At higher redshifts, little is known about the number density of Compton thick AGNs Mid IR + radio selection of type-2 QSOs at z~2 implys twice as large number density as Compton thin type-2 QSOs (Martines-Sansigre et al. 2006)

Evidence for Compton thick AGNs in the local universe A population of infrared galaxies that do not show AGN signatures in their optical spectra are found to be Compton thick AGNs by Chandra follow-up (optically elusive AGNs) The number density is comparable to that of Seyfert 2 galaxies Their nucleus is completely hidden so that no narrow-line region form? Swift/BAT or INTEGRAL surveys will give a definete answer for this at least for those with log 24 <NH<25 Maiolino et al. (2003)

log N log S relation above 10 keV If we simply extrapolate the NH function below log NH=24 to log NH=26 based on the Ueda 2003 model, then The fraction of Compton thick AGNs is predicted to be ~10% (Fx=1e-11) to ~25% (Fx=1e-16) NeXT limit ~40-50% XRB 2-8 keV Survey 10-30 keV Survey Fraction of Compton thick AGNs Ueda+ 03

Summary There is growing evidence for the presence of a large population of Compton thick AGNs in the universe. We have not fully understand the XRB origin yet. The population synthesis model is being almost established below 6-8 keV. However, we have to note that there are a few critical assumptions are made when extrapolating it to above 8 keV. To fully understand the accretion history of the universe, it is critical to reveal the evolution of Compton thick AGNs. Sensitive hard X-ray surveys above 10 keV with various depths and widths, as done at energies below 10 keV, are the only way to unveil this problem.